September 16, 2005

Study Links Hurricanes to Global Warming

Fiercer storms may be due to greenhouses gases, but more data needed, scientists say

An increase in the ferocity of hurricanes around the globe over the last 35 years may be attributable to global warming, a new report states.

The study, which appears in the Sept. 16 issue of the journal Science, is perhaps one of the strongest scientific statements yet on a connection between hurricane activity and global warming.

"I'm heading towards being a little less cautious," study lead author Peter J. Webster, professor at the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology in Atlanta, said at a news conference Wednesday. "I think [rising] sea surface temperature is a global-warming effect and I think the change in [hurricane] intensity, which is a universal thing, is following sea surface temperature."

Webster was referring to a demonstrated increase in the sea surface temperature (SST) of about half a degree centigrade since 1970. Scientists have hypothesized that higher sea surface temperatures result in greater hurricane intensity.

Not everyone is convinced by the new findings, however.

"The question is, is [the increase in intensity] real?" said Chris Landsea, a meteorologist with the National Hurricane Center in Miami. "Are we seeing a big increase the last 15 years or is it an artifact of the data? I'm afraid it's probably not a real change that's going on."

Even one of Webster's co-authors, Greg Holland, director of the Mesoscale and Microscale Meteorology Division of the National Center for Atmospheric Research in Boulder, Colo., hedged his bets a little. "There is a reasonable chance that this is consistent with global change but one can never say for sure with this amount of data," he said.

All of which reflects the ongoing debate in the scientific community as to whether changes in hurricane tempers are due to natural variability or to the effects of global warming.

The Science article comes as U.S. rescue efforts continue in the Gulf Coast areas devastated by Katrina, a category 5 hurricane that battered parts of Louisiana -- most notably New Orleans -- and Mississippi and Alabama earlier this month. The authors of the study said the fury of Katrina on its own, however, cannot specifically be pinned on global warming.

"Katrina was one of those we've seen increasing in intensity but we can't say Katrina by itself was part of this factor," Holland said. "There is a substantial amount of natural variability."

The study authors analyzed the frequency, duration and intensity (maximum wind speed) of hurricanes over the past 35 years in the five major ocean basins. The time period 1970 to 2005 was chosen because equivalent data was available for all years.

The number and frequency of hurricanes grew until 1995, then fell after that, leaving the overall rate steady.

The largest increases in intensity occurred in the North Pacific, Indian and Southwest Pacific Oceans, while the smallest percentage increase occurred in the North Atlantic.

"In all basins including the Atlantic, category 4 and 5 hurricanes have increased enormously, almost by a factor of two," Webster said. "It's not too far from the imagination to be able to ascribe changes in hurricane intensity to SST [sea surface temperatures]."

Hurricanes are rated on a scale of one to five, based on the Saffir/Simpson Hurricane Scale. A category 1 storm has winds ranging from 74 to 95 mph; a category 5 hurricane has winds exceeding 155 mph.

The number of category 1 hurricanes remained about the same, the study found, while the most severe hurricanes have not become any more intense.

Landsea contested some of the data and some of the findings.

"At the start of the study, in 1970, there was no way to even estimate what the winds were of hurricanes over open oceans," he said.

And information on the Atlantic, where planes have been flying since the 1960s, should be the most reliable and that's showing the smallest change of all six ocean basins, he pointed out.

Also, Landsea said, it makes no sense that there would be more category 4 and 5 storms yet no change in peak winds. "Other studies suggest that if global warming is going to have an impact, that the strongest hurricanes will get even stronger and we're not seeing that," he said.

According to Landsea, one of the best studies on what might happen in the future suggests that, in 100 years, a doubling of carbon dioxide and methane in the atmosphere and a 3 to 4 degree warming of the ocean temperature would cause an increase in winds and overall rainfall on the order of only 5 percent.

"The global warming impacts are so tiny today that they can't be measured although they might be measured in 100 years," Landsea said. "Compared to the natural swings of hurricane activity and compared to the huge population increase and infrastructure build-up along the coast, any global warming effects are likely to be so tiny that they're lost in the noise."

But the study authors dispute such thinking.

"We do see this trend in SST that's relentlessly rising and the hurricane intensity that's relentlessly rising. So, with some confidence, we can say that these two things are connected and there's probably a substantial contribution from greenhouse warming and not just a natural variability," said Judith Curry, another co-author and chairwoman of Georgia Tech's School of Earth and Atmospheric Sciences.

"Even with imperfect data and some uncertainty, it's hard to imagine what kind of errors might be in the data set to give you a long-term trend."

Webster added: "The National Weather Service did one heck of a job in forecasting Katrina and, with all the problems that we have with the response of FEMA and so on, we sometimes forget we do something with hurricanes very well. There was an enormous warning given to the region."

More information

The National Oceanic and Atmospheric Administration (www.gfdl.noaa.gov ) has more on global warming and hurricanes.